Lymphopoiesis is the process by which lymphocytes are generated from pluripotent hematopoietic stem cells. I am interested in defining the role of two Ets domain transcription factors, PU.1 and Spi-B, in the differentiation and function of B lymphocytes. Using gene knockout technology in mice, we have previously shown that PU.1 is an essential protein for embryonic development in mice. PU.1 minus/minus mice lack both lymphoid and myeloid mice cells and die in utero. In contrast, Spi-B minus/minus mice are viable but exhibit defects in the functional response of B cells to antigen. To dissect both the unique and redundant functions mediated by Spi-B and PU.1 more specifically, I have generated mice with the genotypes PU.1 plus/minus, Spi-B plus/minus, PU.1 plus/minus, Spi-B minus/minus and PU.1 minus/minus, Spi-B minus/minus. PU.1 minus/minus, Spi-B minus/minus mice exhibit the same phenotype as PU.1 minus/minus, Spi-B plus/plus mice: they lack all lymphoid and myeloid cells and die in utero. Interestingly, PU.1 plus/minus, Spi-B minus/minus mice are viable but exhibit greatly decreased numbers of B220+/IgM+ and IgM+/IgD+ immature and mature B cells in the bone marrow and peripheral lymphoid organs. These mice also exhibit severe defects in mature B cell function. My preliminary data strongly suggest that signaling via the B cell receptor (BCR) is defective in Spi-B minus/minus and PU.1 plus/minus, Spi- B minus/minus mice at a membrane-proximal step prior to the activation of protein kinase C (PKC). I propose experiments to (1) characterize the B cell developmental defect in PU.1 plus/minus, Spi-B minus/minus mice and (2) identify the molecular basis of the BCR signaling defect in Spi-B minus/minus and PU.1 plus/minus, Spi-B minus/minus mice.